Felsic Research Articles

Magma ascent and degassing processes of the 2011 and 2017–18 eruptions of Shinmoedake in Kirishima volcano group, Japan, based on petrological characteristics and volatile content of magmas

The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m−3) than the felsic magma (2264–2496 kg m−3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase, clinopyroxene, and orthopyroxene phenocryst cores from 2018 and 2011 were similar, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since February 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion.Graphical

Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas

ABSTRACT The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings.

U–Pb Age of Detrital Zircon in the Cement of Early Precambrian Polymict Conglomerates of the Central Karelia Domain, Karelian Province, Fennoscandian Shield

U–Th–Pb LA-ICP-MS was used for dating detrital zircon extracted from the cement of Neoarchean and Paleoproterozoic polymict conglomerates of the Karelian Province, Fennoscandian Shield. The chemical composition of terrigenous and volcanogenic rocks of the Gimolskaya Formation is presented for the first time. The data obtained showed that the detrital zircon from the cement of conglomerates of different ages yields similar results of about 2750 Ma. The clasts of the gneiss tonalite and plagioporphyry dominate in the Neoarchean conglomerates of the Sukkosero area. They are identical in chemical composition with the Neoarchean (2.78–2.73 Ga) TTG, granodiorite, and metadacite of the Central Karelian domain. These rocks are distinguished by an increased content of Ba (900–1200 ppm) and Sr (600–700 ppm), and the ratios Sr/Y = 34–90 and (La/Yb)n = 17–30. The chemical composition of the cement (values of CIA < 55 and negative values of the function DF(x)) from –1.0 to –0.4 is similar to the composition of clasts, which means the cement is composed of finer clasts of the same rocks. The Paleoproterozoic conglomerates of the Vottomuks area contain zircon from felsic rocks of the Neoarchean basement. The chemical composition of the cement of the Paleoproterozoic conglomerates and the predominance of basaltic andesites of Sumian are evidence of the post-Sumian(<2.45 Ga) age of these rocks. The absence of the Paleoproterozoic zircon in cement could have resulted from its absence in the Paleoproterozoic source rocks. The age of Neoarchean conglomerates from the Gimolskaya Formation is less than 2.75 Ga.

Origin of Amphibole-Biotite-Fluorite-Rich Enclaves from Gabal El-Ineigi Fluorite-Bearing Granite, Central Eastern Desert of Egypt: Insights into Fluoride–Calcium and Silicate Liquid Immiscibility

Gabal El-Ineigi fluorite-bearing rare-metal granite with A-type affinity, located in the Central Eastern Desert of Egypt, is distinguished by its abundance of large fluorite-quartz veins and mafic enclaves. Plagioclase (labradorite to oligoclase), Mg-rich biotite, and Mg-rich hornblende are the main components of mafic enclaves, with significant amounts of fluorite as essential phases, and titanite and Fe-Ti oxides (Nb-free rutile and ilmenite-rutile solid solution) as the main accessories. These enclaves are monzodioritic in composition, Si-poor, and highly enriched in Ca, Fe, Mg, and F compared to the host alkali feldspar F-poor Si-rich granites. Given the conflicting evidence for a restitic, xenolithic, magma mixing/mingling, cumulate, or bimodal origin for these enclaves, we propose that the mafic enclaves and felsic host granites are two conjugate liquids, with contrasting compositions, of a single parental melt. This is inferred by the normalized REE patterns that are similar. As a result, liquid immiscibility is proposed as a probable explanation for this mafic–felsic rock association. These enclaves can be interpreted as transient melt phases between pure silicate and calcium-fluoride melts that are preserved from the early stages of separation before evolving into a pure fluoride (Ca-F) melt during magma evolution. Due to element partitioning related to melt unmixing, the enclaves are preferentially enriched in Ca, F, Li, Y, and REE and depleted in HFSE (such as Zr, U, Th, Ta, Nb, Hf, and Ga) in comparison to the host granites. Furthermore, mafic enclaves exhibit W-type tetrad effects, while host granites exhibit M-type tetrad effects, implying that the REE partitioning, caused by liquid immiscibility, is complementary.

Crustal recycling and growth via mélange diapir in subduction zones: Insights from two episodes of magmatism in the Northern Yili Block, NW China

Arc magmatism can provide crucial information about crustal recycling and growth in subduction zones. However, the crustal material transfer process and mechanism from subducting slab to overlying mantle wedge are controversial. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf isotopic data, and previously published data for two episodes of subduction-related magmatism in the Northern Yili Block, NW China, to study these issues in relation to the recycling of continental crust and the growth of two separate magmatic episodes at 400−350 Ma and 350−300 Ma, respectively. The Nd-Sr isotope modeling results, low Nd/Sr, and variable Hf/Nd ratios demonstrate that the arc magmatism of these two episodes could be derived from two distinct mélange sources that formed at the slab-mantle interface during subduction of the Junggar oceanic plate. In the first episode, magmatic rocks exhibit high Th/Yb and (La/Sm)N ratios and the decoupling of Nd-Hf isotopes, which leads to an interpretation that the primary magmas were derived from sources mainly mixed by sediments and mantle-wedge peridotites. In contrast, the magmas of the second episode exhibit high Ba/La and Ba/Th ratios and the coupling of Nd-Hf isotopes, which implies that these magmas were likely produced from mélange diapirs dominated by the mixing of mid-oceanic-ridge basalts and mantle peridotites. The episodes of Nd-Hf isotopic decoupling and coupling coincided with crustal material recycling and crustal growth in subduction zones, respectively. Considering that the Northern Yili Block at 400−350 Ma was characterized mainly by enriched Nd-Hf isotopes of magmatic rocks and no development of the forearc accretionary complex, while the Northern Yili Block at 350−300 Ma featured high εHf(t)-εNd(t) values of magmatic rocks, occurrences of extension-related magmatism, and distribution of the forearc accretionary complex and immature back-arc basin, we propose that these two different mélange sources probably developed in response to subduction transition. That is, one resulted from advancing and the other was relevant to retreating regime. The partial melting of these mélanges in the mantle wedge generated intermediate felsic magmas, which might have acted as a leading mechanism for crustal recycling and growth of the accretionary orogenic belt.

The ~1.1 Ga St. Ignace Island complex, Northern Ontario, Canada: Evidence for magma mixing and crustal melting in the generation of Midcontinent Rift-related bimodal magmas and implications for regional metallogeny

Abstract The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is a ~26 km2 stock with a felsic core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107±8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032-0.7045) and 143Nd/144Ndi (0.51051-0.51057) whereas the mafic end members have similar 87Sr/86Sri (0.7040-0.7061) but more radiogenic 143Nd/144Ndi (0.51067-0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu-(Ag) deposits within the rift and the presence of small porphyry-style deposits.

Petrographic features of Late Cretaceous biotite granites in the Kurtoğlu region (Elazığ/Turkey)

The study area is located in a local area around Kurtoğlu village of Elazığ province, located within the Southeast Anatolian Orogenic Belt. The studied biotite granites belong to the Elazig Magmatic Complex. The Elazığ Magmatic Complex is grouped into volcanic, sub-volcanic and plutonic rocks and mafic and felsic. Felsic rocks belonging to the Elazığ Magmatic Complex are represented by granite, granodiorite, tonalite, quartz monzonite, monzodiorite, and mafic rocks are represented by diorite, quartz diorite and gabbros. The biotite granites are porphyric biotite minerals with an average size of one cm. Petrographically, they are composed of K-feldspar, plagioclase, quartz, biotite and opaque minerals. K-feldspars have low birefringence colours, grey tones, and earthy colours in single nicol. Plagioclases; anhedral, euhedral and mostly subhedral crystals. They typically indicate albite, albite+karlsbad and polysynthetic twinning. In some samples, sericitization and carbonation are observed mostly in the middle parts of the plagioclase resulting from alteration. Biotites; It is generally in the form of subhedral platy-prismatic, rod-like crystals. Pleochroism is seen in brown tones in single nicol. It has high birefringence colours and indicates vivid interference colours in yellowish, blue and green, especially brown tones in double nicol. Porphyritic texture in which phenocryst and smaller crystals are generally observed in granites.

A new type of saidenbachite with pseudomorphs after coesite phenocrysts from the north‐western Bohemian Massif, Germany

AbstractSaidenbachite started to crystallize from melt under ultrahigh pressure (UHP). This felsic igneous rock is known from two occurrences in the world only. The here reported new type of saidenbachite, found at the type locality in the central Erzgebirge Crystalline Complex, is characterized by millimetre‐sized aggregates of coarse‐grained quartz in a matrix of significantly smaller grains of garnet, rarely containing diamond inclusions, plagioclase, K‐feldspar, white mica, some dark mica, rutile and ±kyanite. According to the shape and the regular distribution of these aggregates showing similar size, they are interpreted as former coesite phenocrysts, which grew early in a felsic magma. This interpretation is corroborated by thermodynamic modelling demonstrating that coesite is the first major phase that crystallizes from SiO2‐rich (&gt;66 wt%) magmas at UHP.

Geochemical characterisation of clastic sediments in Kompina (N'kapa Formation, NW Douala Basin, West Cameroon): Implication for provenance, paleoweathering and sediment maturity

Chemical analyses were carried out on clastic sedimentary rocks in Kompina (N'kapa Formation of the NW, Douala Basin, West Africa) to disclosed the composition of their source rock, characterised their tectonic domains, decipher the intensity of the past weathering, sedimentary cycles and maturity using concentrations of major oxides, REES and trace elements. Provenance diagram constructed from ratios of La/Co, La/Sc, Th/Sc, Cr/Th and from binary diagrams of Zr vs TiO2 and AL2O3vs TiO2, disclosed a felsic rock composition as the source rock of the Kompina clastic rocks. The felsic source rock composition designated for the studied clastic materials is also supported by LREE enrichment over HREE and a negative europium anomaly on chondrite calculation and diagram. New discriminant functions diagrams to delineate between active and passive domains such as DF 1&2(Arc-Rift-Col)M1, DF1&2(Arc-Rift-Col)M2 combined with diagrams of DF(A-P)M, DF(A-P)MT label a passive tectonic setting characteristics for the source rock where there studied clastic materials were sorted. The weathering intensity and plagioclase lixiviation revealed by the CIA and PIA indexes advocate a weak to intense strength of chemical weathering and lixiviation of plagioclase felspars while the CIX and PIX with elimination of CaO in their formulars show extreme intensity of weathering and lixiviation of plagioclase felspars.Most of the samples show immature nature from their ICV values > 1 but with the introduction of ICVnew in this work, where oxides of iron and calcite are considered as cement and eliminated from the formular show that all the studied samples have values < 1 indicating they are mature. Plotted diagrams of Th/Sc, and (Gd/Yb)N ratios, with relationship of Zr and (La/Yb)N shows that the studied clastic materials are mature, second cycle sediments, which have experience zircon mineral addition.

Petrogenesis and geochemical evidence from early Jurassic intrusive rocks and their implications for subduction of the Paleo-Pacific Plate in the Yanbian area, Northeast China

Subduction of the Paleo-Pacific Plate plays a key role in the Early Jurassic magmatism evolution in the Yanbian area, Northeast China. In this paper, zircon U–Pb ages, whole-rock geochemical, and Sr–Nd–Hf isotopic compositions are presented. The Early Jurassic gabbros, diorite enclaves, granodiorites, and granites ages range from 183 to 189 Ma. They are characterized by enrichment in large-ion lithophile elements and depletion in high-field strength elements. The gabbros are classified as low- and medium-K tholeiitic, granodiorites and granites are high-K calc-alkaline I-type, and diorite enclaves are calc-alkaline transitional series. The gabbros have zircon εHf(t) values of 6.81–9.89, whole-rock 87Sr/86Sr(i) = 0.7043–0.7044, and εNd(t) from 2.72 to 2.80. The diorite enclaves have zircons with εHf(t) values of 8.26–12.80, whole-rock 87Sr/86Sr(i) = 0.7051, and εNd(t) from 0.96 to 1.09. The granodiorites and granites have zircon εHf(t) values of 7.59–12.87, whole-rock 87Sr/86Sr(i) = 0.7042–0.7066, and εNd(t) from 2.33 to 2.61. These data indicate that gabbroic magmas were derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The basaltic magma underplated and heated the juvenile crust, whereas the granodiorites and granites might be the product of fractional crystallization of a mixture of basaltic and felsic magmas or derived from partial melting of the juvenile lower crust. Our data constrain the petrogenesis of these Early Jurassic intrusive rocks which are attributed to subduction of the Paleo-Pacific Plate beneath Eurasia continent in northeastern China.

Middle Triassic to Jurassic convergence at the north-western margin of Gondwana: insights from the Central Cordillera of Colombia

ABSTRACT The northern Andes are crucial for understanding the processes associated with the break-up of Pangea and its impact on the evolution of the Pacific margin of South America. Despite its importance, the origin and significance of mafic magmatism and its relationship with the Permian arc-related granitoids in the northern Andes remains controversial. To address this, we examined the Tierradentro gneisses and amphibolites, which comprise Permian-Triassic lithologies that may help resolve this controversy. Our study focuses on representative lithologies of the Central Cordillera of Colombia, including mylonitized granitoids, metabasites, metapelites, and serpentinites, to shed light on the history of the western margin of Gondwana during and after its break-up (~230 to 200 Ma). To investigate these lithologies, we conducted field observations, petrographic analyses, U-Pb dating of zircons, and whole-rock geochemical, Nd, Pb, and Hf isotopic analyses. Our results show that mylonitized mafic rocks with Triassic crystallization ages (ca. 236 Ma) intruded Permian granitoids. Furthermore, these mafic and felsic rocks were intruded by Jurassic calc-alkaline batholiths. The geochemical and isotopic signatures of these rocks indicate their subduction-related extension origin and further mylonitic deformation, possibly during the Jurassic strike-slip movement of the Farallón plate relative to western Gondwana. The Otú-Pericos fault bounds these rocks to the west, where Jurassic metabasic and metapelitic sequences with serpentinites interleaved and formed in a collisional setting. Our findings suggest that the Pacific margin of northern South America underwent subduction-related extension during the middle Triassic and evolved towards a more oblique convergence during the Jurassic.

Pore structure characteristics of different lithofacies of the Longmaxi shale, Western Hunan‐Hubei Region, China: Implications for reservoir quality prediction

Lithofacies and micropore characteristics of shale reservoir directly affect the exploration and development of natural gas. To determine the development potential of shales of the Longmaxi Formation in the Western Hunan‐Hubei Region, the lithofacies types and micropore structure characteristics of the Longmaxi Formation shales have been systematically studied in this paper, based on ordinary thin sections, argon ion polishing–scanning electron microscopy, physical property testing, x‐ray diffraction whole‐rock and clay mineral analysis, isothermal nitrogen, and carbon dioxide adsorptions. According to the principle of ‘laminar structure + total organic carbon (TOC) + mineral composition of felsic, clay, and carbonate minerals’, eight types of shale lithofacies were identified. The effective porosity of the Longmaxi shale ranges from 1.8% to 14.4%, with a peak range from 3% to 6%, while the permeability ranges from 0.001 to 1.27 mD, with an average of 0.106 mD. Combined with the isothermal adsorption experiments, the proportion of mesopores is the largest in the Longmaxi shale, followed by macropores and micropores. Moreover, the pore diameters of mesopores are concentrated between 3 and 10 nm, while those of micropores are concentrated between 0.4 and 0.9 nm. The pore volume of shale is most developed when the content of brittle felsic minerals is 50% ~ 75% and the content of clay minerals is 25% ~ 50%, and the TOC content is positively correlated with the pore volume and specific surface area. There are significant differences in the pore development characteristics of different lithofacies. The shale reservoir space analysis indicates that micropores are controlled by TOC content and clay minerals, while the macropores are controlled by depositional intergranular pores, TOC content, and felsic content, as well as diagenetic pyrite mould pores, microfractures pores, and intercrystalline pores. On the other hand, mesopores are related to a mix of depositional and diagenetic factors with dominant contribution of intergranular pores, intercrystalline pores, organic pores, and intergranular shrinkage fractures pores. It is found that the pore development of massive organic‐rich siliceous shale lithofacies has the best reservoir quality, including well‐connected intergranular pores, intercrystalline pores, organic pores, and shrinkage fractures. Therefore, the massive organic‐rich siliceous shale lithofacies is the dominant fine‐grained reservoir lithofacies in the studied area. Based on understating the pore characteristics of different lithofacies, this study can provide a scientific basis for the prediction of favourable sweet spots of shale gas reservoir in the Longmaxi Formation, western Hunan‐Hubei Region.

Prediction of lamina structure and reservoir quality in shale using well logs: The Cretaceous Qingshankou Formation, Gulong Sag, Songliao Basin, China

Lamina structure is determined by composition, thickness, continuity, and mineral assemblage. Reservoir quality in oil shale reservoirs is determined by lamina structure, which controls hydrocarbon accumulation and migration. The prediction and evaluation of multi-scale lamina structures using geophysical well logs is challenging. Recent studies demonstrate that the lamina structure classification may be based on core observation and core analysis. However, there are no corresponding high-resolution geophysical logging methods to identify and categorize lamina structure continuously in a single well. Here, an experimental approach, involving an integrated analysis of cores, thin sections, well logs such as slabs and button conductivity curves of image logs (up to 5 mm vertical resolution), is used to characterize lamina structure. T1-T2 (T1, longitudinal relaxation time, and T2, transverse relaxation time) maps of nuclear magnetic resonance (NMR) logs are applied to clarify the relationships between the lamina structure, reservoir quality, and oil-bearing properties. Results demonstrate how lamina structure in shale can be divided into three types: laminated rocks, layered rocks, and massive rocks, according to observation of core at the lamina scale. The thickness of individual lamina is less than 0.01 m in the laminated rocks, and the layered rocks have bundles of genetically related laminae thickness ranging between 0.01 m and 0.1 m. In massive rocks there is no visible layering. The well log response patterns of the three types of lamina structure are established so that the distribution of lamina structure can be predicted in a single well via well logs. Results show that the oil-bearing shale intervals are dominated by laminated rocks and layered rocks, accounting for 90% of the studied Qingshankou Formation. In addition, the layered rocks play an important role in improving reservoir quality and the laminated rocks present moderate reservoir quality. The massive rocks show poor reservoir quality. Moreover, an increased proportion of felsic mineral content favors increased oil potential in oil shale reservoirs. The method is generally applicable and is of value in developing predictive indicators of subsurface reservoir sweet spots in shale oil and gas exploration and development.

A 2600-yr multiproxy record for climate and vegetation reconstruction along the Mahanadi River delta, east coast of India

Understanding the impact of climate change on the vegetation cover in the past is crucial for predicting the future warming scenario and its consequences. The present study integrates biotic (palynological analysis) and abiotic proxy (sediment chemistry, clay mineral assemblages and grain size end member (EM) analysis) records in a chronologically constrained sedimentary sequence from the southeastern part of the Mahanadi River delta to reconstruct the hydroclimate since 2600 years. The Mahanadi delta sediments are primarily derived from felsic rock source possibly composed of granites or granulites. The dominance of herbaceous taxa, high aridity index and deposition of fine grain (EM1) sediment possibly suggest a relatively cold and dry climate in the study region during ~2600–2100 cal yr BP corresponding to the Iron Age Cold Period (IACP). Presence of diverse mangrove and tree taxa, deposition of coarser sediment (EM3) and relatively high abundance of secondary clay minerals possibly indicate high precipitation associated with the Roman Warm Period (RWP, ~2100–1800 cal yr BP). A decline in mangroves, dominance of herbaceous taxa and high deposition of fine grain (EM2) sediment from ~1800 to 1300 cal yr BP may indicate a relatively drier climate coinciding with the Dark Age Cold Period (DACP). High moisture index and an increased value of Rb/Sr during ~1300 cal yr BP may suggest a wet phase with the onset of the Medieval Climatic Anomaly (MCA) and later shifted to an arid climate during ~1100–800 cal yr BP. After ~800 cal yr BP, a sharp decline in all weathering indices and vegetation cover, dominance of illite indicates a relatively dry climate which could mark the Little Ice Age (LIA, ~800–300 cal yr BP). A small increase in the terrestrial flux, as well as the weathering indices after ~300 cal yr BP, may suggest the onset of recent warming.

Rubidium Isotope Ratios of International Geological Reference Materials

In this study we determined rubidium isotope ratios in twenty‐one commonly used international geological reference materials, including igneous, sedimentary and metamorphic rocks, as well as an IAPSO seawater reference material. All δ87Rb results were obtained relative to the NIST SRM 984 reference material. For most reference materials, Rb was purified using a single column loaded with Sr‐spec resin. For reference materials containing low Rb but high mass fractions of matrix elements (such as basic rock and seawater), Rb was purified using two‐column chromatography, with the first column packed with AGMP‐50 resin and the second column packed with Sr‐spec resin. Two methods for instrumental mass bias correction, sample‐standard bracketing (SSB) mode, and the combined sample‐standard bracketing and Zr internal normalisation (C‐SSBIN) method, were compared for Rb isotopic measurements by multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS). The long‐term reproducibility of Rb isotopic measurements using both methods was similar, better than 0.06‰ (2s, standard deviation) for NIST SRM 984. Significant Rb isotopic fractionation was observed among the reference materials, with an overall variation in δ87Rb values of approximately 0.5‰. The δ87Rb values of igneous rocks ranged from ‐0.28‰ to +0.06‰, showing a trend from heavier isotopic compositions in mafic rocks to lighter δ87Rb values in the more evolved felsic rocks. The sedimentary and metamorphic rocks had Rb isotope ratios similar to those of igneous rocks. The δ87Rb values of the reference materials related to low‐temperature geological processes showed a wider range than those of high‐temperature processes. Notably, the IAPSO seawater reference material had a δ87Rb value of +0.14‰, which deviated from that of igneous rocks, and represents the heaviest reservoir of Rb isotopes found thus far on Earth. The comprehensive dataset presented here has the potential to serve for quality assurance purposes, and provide a framework for interlaboratory comparisons of Rb isotope ratios.

Provenance Analysis of the Ojén Nappe and Its Implication for the Geodynamic History (Alpujárride Complex, Betic Cordilleras, Spain)

The Ojén nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras), is composed of Triassic marbles overlying a metapelitic sequence. In order to carry out an LA-ICP-MS U-Pb age determination and provenance analysis, detrital zircon grains from two quartzites interlayered within marbles and metapelites were extracted. The obtained results yield the youngest zircon population of 254 ± 3 Ma (Late Permian) with a mean Th/U ratio of 0.34 that supports both the Permian–Triassic age of the marble member and the felsic magma nature for zircon-bearing protoliths. The Permian zircons exhibit tight age peaks in three main populations of 292 ± 2 Ma, 278 ± 3 Ma, and 254 ± 3 Ma, in agreement with the main age populations reported from rhyolites and shallow crustal basaltic–andesite subalkaline rocks emplaced in transtensional Permian basins of the Variscan Orogen during the break-up of Pangea. Moreover, the analyzed detrital zircon grains yield age distributions with clear Tonian–Stenian (ca. 950–975 Ma), Ediacaran–Cryogenian (ca. 560–615 Ma), Ordovician (ca. 460–465 Ma), and Cisuralian peaks (ca. 280–290 Ma) and smoothed Mesoproterozoic peaks that point to a weak and residual Cadomian peri-Gondwanan terrane inheritance. The identified Middle Ordovician detrital zircon populations (460–465 Ma) strengthen the hypothesis that the Alborán domain would be located along the southern passive margin of the European Hunic superterrane.

Mixing of magmas from multiple sources in the petrogenesis of multi‐stage Early Cretaceous syenites in the Wulingshan alkaline complex, northern North China Craton: Evidence from enclaves

There are several different models for the origin of syenites, but the role of magma mixing in the formation of syenites remains unclear. The Wulingshan alkaline complex in the northern North China Craton consists mainly of porphyritic syenite and syenite with abundant enclaves. These enclaves may provide new insights into the petrogenesis of syenites. We obtained zircon U–Pb age, mineral chemistry, whole‐rock major and trace elements, and Sr–Nd isotopic data for the enclaves and their host rocks to constrain the petrogenesis and identify the role of magma mixing during the formation of the different syenites in the Wulingshan alkaline complex. The results of zircon U–Pb dating indicate that the enclaves and host rocks crystallized contemporaneously at ca. 133 Ma. The enclaves contain abundant clinopyroxene, amphibole, and biotite, and their average Nb/Ta (18.46) and Th/Ce (0.04) ratios are similar to those of the mantle. The enclaves have relatively high Fe2O3T, MgO, and CaO contents, and their SiO2contents are equivalent to those of intermediate rocks, indicating that they formed by mixing the mafic and felsic magmas to some extent. The different mineral assemblages, major and trace elements and isotopic compositions of the enclaves in the porphyritic syenite and syenite indicate that they are two batches of parental magma with different properties derived from the mantle. The porphyritic syenite and syenite have high SiO2and low Fe2O3T, MgO, and CaO contents, as well as low V, Cr, Co, and Ni contents. These major and trace element characteristics of the host rocks require the involvement of crustal components. The Sr–Nd isotopic compositions of the enclaves and the host syenitoids plot on a mixing curve between enriched lithospheric mantle and lower crust, indicating that these rocks are the products of magma mixing and crust–mantle interaction. Considering the contact relationships, the geochemistry data, and the mixing model presented in the paper, we propose that the upwelling of the hot asthenosphere heated the overlying enriched lithospheric mantle and triggered low‐degree partial melting. The alkalic mafic magmas derived from enriched mantle sources were mixed with felsic magmas generated by partial melting of the lower (or upper) crust to form the different syenites.

Magma-mixing origin for the Early Jurassic intrusive rocks in the eastern Songnen–Zhangguangcai Range Massif, NE China: Evidence from geochronology, geochemistry and Hf–O–Sr–Nd isotopes

Granites containing abundant dioritic enclaves are widespread in the Songnen–Zhangguangcai Range Massif (SZRM). However, the role of mantle- and crust-derived magmas in the petrogenesis of the dioritic enclaves is debated. We present new zircon U–Pb–Hf–O isotopic compositions, whole-rock geochemistry, mineral geochemistry, and Sr–Nd isotopic compositions of hornblendites and dioritic enclaves and their host granites to elucidate their origins and investigate crust–mantle hybridization. Zircon U–Pb dating suggests that these intrusive rocks were emplaced during the Early Jurassic (197–186 Ma). The geochemical characteristics of hornblendites are similar to those of the Early Jurassic mafic–ultramafic rocks in the Lesser Xing’an–Zhangguangcai Range. They yield low (87Sr/86Sr)i ratios (0.704266–0.704324), high εNd(t) values (3.87–3.91), and are enriched in Ba, Sr, and K and depleted in Nb, Ta, and Ti, suggesting that they were derived from the partial melting of mantle wedge metasomatized by sediment melt. The host granites have high SiO2 (>65.12 wt%) and low MgO (<1.92 wt%) and Al2O3 (<16.23 wt%) contents and are to metaluminous–peraluminous high-K calc-alkaline rocks. In addition, they yield positive zircon εHf(t) values (3.42–7.58), zircon δ18O values (4.87‰–5.85‰) and whole-rock εNd(t) values (0.10–0.44), suggesting that the granitic magma originated from the partial melting of continental crust with the involvement of mantle-derived melts. The field and petrographic observations and geochemical analyses indicate that the enclaves were derived from magma mixing and mingling. The dioritic enclaves contain K-feldspar megacrysts and back-veins, and plagioclase crystals within them are reversely zoned and have ocellar textures. The weighted mean ages of the host granites (186 ± 3 and 188 ± 2.3 Ma) are similar to those of the dioritic enclaves (186 ± 1.8 and 188 ± 1.1 Ma). The major element contents of the intrusive rock are linearly correlated with the SiO2 contents, and the whole-rock geochemical and isotopic characteristics of the hornblendites and host granites show that they are the mafic and felsic end-members, respectively, involved in the magma mixing. Therefore, we propose that underplating by mantle-derived magma likely triggered partial melting of the continental crust to produce felsic magma, and the dioritic enclaves were formed by the mixing and mingling of mantle- and crust-derived magma alongside melting, assimilation, storage, and homogenization. This suggest that the Mudanjiang Oceanic crust was subducted westward beneath the SZRM during the Early Jurassic.

Petrogenesis of late Jurassic Mufushan high-Mg diorites and late Mesozoic tectonic evolution of the eastern South China Block

Abundant Mesozoic magmatic rocks are widely distributed in southeastern China, forming several remarkable magmatic-metallogenetic belts. However, the geodynamic mechanism for the Late Jurassic-Early Cretaceous magmatic processes and their relation to the rich polymetallic deposits of the region remain highly controversial. Here, we report zircon U-Pb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the high-Mg diorites in the Mufushan region, northeastern Hunan Province. The high-Mg diorites were emplaced in the late Jurassic (U-Pb ages of ∼151–155 Ma), contemporaneous with the surrounding granitic rocks which have ages ranging from 140 to 150 Ma. These high-Mg diorites are characterized by the enrichment of large-ion lithophile elements (LILEs, e.g. Ba, Pb, K and Rb) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Ti), displaying typical geochemical features of continental-arc basalts (CAB). The relatively high Mg# values (71–76, average of 72), Cr contents (546–1562 ppm, average of 944 ppm) and Ni contents (211–519 ppm, average of 335 ppm) suggest that their derivation was likely from a relatively primitive magma, accompanied with weak crustal assimilation or fractional crystallization.The high-Mg diorites display isotopic characteristics similar to those of enriched mantle-derived mafic rocks, such as low 87Sr/86Sr(i) (0.70692–0.70789), εNd(t) and εHf(t) ranging from −2.4 to −4.3 and −3.8 to + 1.1 respectively, with single-stage Nd model ages (TDM1) of 1.3–1.5 Ga. In combination with a compilation of coeval magmatic rocks in eastern South China, a significant addition of ancient lithospheric mantle materials was likely involved in the Mesozoic magma genesis. The decreasing radiogenic Nd isotope compositions and negative Hf signatures suggest an increasing contribution of enriched components to the magma source from the Late Jurassic to Early Cretaceous. Our results suggest that the Mufushan high-Mg diorites were derived from mantle-derived magma, followed by the incorporation of slab-derived materials into mantle and subsequent sediment melting. Integrated with previous work, the widespread Late Jurassic-Early Cretaceous magmatic rocks in eastern South China formed within a continental back-arc setting that was formed by subduction-induced convection in the mantle wedge. The subsequent roll-back of subducted Paleo-Pacific slab (Izanagi plate) triggered extensive mantle upwelling and melt-mantle interaction, accounting for the production of contemporaneous felsic rocks and related polymetallic deposits in regions.

Pore Distribution Characteristics of Different Lithofacies Shales: Evidence from Scanning Electron Microscopy

To disclose the pore distribution characteristics of different lithofacies lacustrine shales, ten samples collected from the Shahejie Formation, Dongying Sag, Bohai Bay Basin, China, were examined using argon ion beam milling–scanning electron microscopy (SEM). A quantitative method was adopted to characterize shale pore distributions based on the SEM images. Mercury intrusion capillary pressure was employed to determine the pore throat size distributions of the shales. The SEM images indicated that in shale reservoirs, interparticle pores at the edges of brittle particles and intraparticle pores in clay mineral aggregates primarily contribute to the reservoir spaces and that in calcite-rich shales, dissolution pores provide secondary reservoir space. Among the morphologies of dissolution, intraparticle, and interparticle pores, the morphology of the dissolution pores is the simplest, followed by those of intraparticle and interparticle pores in that order. Clay and felsic minerals primarily control the shale pore sizes and the larger the clay mineral content in the shales, the smaller their pore size; the felsic minerals demonstrate the opposite behavior. The image-based point counting data indicate that shale pore sizes are mostly between 20 nm and 2 μm. In contrast, most pore throats are less than 20 nm in diameter, implying that the pores in the nanometer to micrometer scales are connected by tiny throats. Among the four lithofacies shales, felsic-rich shales are favorable for shale oil accumulation and seepage because of their large pore sizes and throats their ability to form intercalated shale oil adjacent to organic-rich shales. Calcareous shales with a large number of dissolution pores and a large content of organic matter could produce self-generation and self-storage matrix shale oil and would typically develop fractures, thereby creating a seepage channel for shale oil. This study presents the micro-distributions of different lithofacies of shale pores, which would help in understanding the occurrence and seepage of oil in shale reservoirs.